How Poison Ivy Works

According to the American Academy of Dermatology, an estimated 10 to 50 million people in this countr­y ­have an allergic reaction to poison ivy each year [ref]. Poison ivy is often very difficult to spot. It closely resembles several other common garden plants, and can also blend in with other plants and weeds. But if you come into contact with it, you'll soon know by the itchy, blistery rash that forms on your skin.

In this article, you'll discover how poison ivy causes that rash, learn where it grows, how to get rid of it and how to spot it before you get too close.

Up Next

Poison Ivy Basics

­Poison ivy is a red, itchy rash caused by the plant that bears its name. Many people get it when they are hiking or working in their garden and ­accidentally come into direct contact with the plant's leaves, roots, or stems. The poison ivy rash often looks like red lines, and sometimes it forms blisters.

The culprit behind the rash is a chemical in the sap of poison ivy plants called urushiol (oo-roo-shee-ohl). Its name comes from the Japanese word "urushi," meaning lacquer. Urushiol is the same substance that triggers an allergic reaction when people touch poison oak and poison sumac plants. Poison ivy (Toxicodendron radican), Eastern poison oak (Toxicodendron quercifolium), Western poison oak (Toxicodendron diversilobum), and poison sumac (Toxicodendron vernix) are all members of the same family -- Anacardiaceae.

About 85 percent of people are allergic to the urushiol in poison ivy, according to the American Academy of Dermatology [ref]. Only a tiny amount of this chemical -- 1 billionth of a gram -- is enough to cause a rash in many people. Some people may boast that they've been exposed to poison ivy many times and have never gotten the rash, but that doesn't necessarily mean they're not allergic. Sometimes the allergy doesn't emerge until you've been exposed several times, and some people develop a rash after their very first exposure. It may take up to ten days for the rash to emerge the first time.

We'll take a closer looks at some of the different types of poison plants on the next page.

If the animation above isn't working, click here to get the Shockwave player.